The present invention relates to an optical telecommunications network, and a method of routing packets carried on such a network.
Conventionally, telecommunications networks, whether based on copper wires or optical fibres, have been organised on the basis of the provision of circuits between customers--either semi-permanent "private" circuits or temporary "dial-up" circuits. Until recently, the traffic carried by such circuits comprised primarily voice telephony together with some low-speed data traffic. Although the replacement of copper fibre networks with optical fibres has provided higher transmission bandwidths and opened the way to the use of the network for high data-rate transmissions and services such as video telephony, hitherto network resources have been allocated to such services still on the basis of circuit provisions. However this approach becomes increasingly inadequate in terms of efficiency of use of the available bandwidth, and network load management, as the traffic carried becomes increasingly bursty, wide-ranging and rapidly fluctuating in bandwidth requirement, with widely diverse message destinations and low predictability. In particular, under these conditions, it becomes increasingly difficult to manage centrally the allocation of specific transmission routes to respective circuit connections.
The paper by Bononi et al. published at pp 2166-2176, Journal of Lightwave Technology, Vol. 11, No. 12, December 1993 discloses a method of routing packets on an optical network based on header recognition. The header of an incoming packet is demultiplexed using an optical AND gate and converted into the electrical domain by photodetectors. The header is then processed in the electrical domain and a routing decision made based on recognition of the address carried in the header. Since this operation is carried out in the electrical domain and involves complex logical operations on lengthy multi-bit addresses, the switching operation is necessarily slow, and potentially represents a serious bottleneck in the optical network.
The papers by Islam et al. published in IEEE Journal of Quantum Electronics 27(3) 843-848 (1991) and Journal of Lightwave Technology 11(12) 2182-2190 (1993) describe a ring network in which logic operations are carried out on packet headers in the optical domain using soliton-based fibre logic gates. Although this avoids some of the bandwidth limitations of the Bononi system, the system again relies upon address recognition as the basis of the routing decision. In the example described, the routing decision is limited to determining whether or not a given packet carries the local address of the node, and so the module is not able to route intelligently packets not addressed to that node. Even this limited routing ability requires the processing of a full address, and more complex routing decisions would require the storing and processing of many such addresses.
The paper by Prucnal et al, published in Optical Engineering 29 (3) 170-182 (1990) discloses an optical routing controller which, as in the prior art systems discussed above, relies upon address recognition. The controller reads destination addresses and appropriately sets a photonic switch using an optical look-up table. That look-up table has to carry as many header words as there are addresses on the network, and so for a realistically large network is likely to be undesirably complex in structure and operation.